INTRODUCTION 7 



Rather curious things happen if no rotation is followed, 

 but the same crop is grown on the land year after year without 

 intermission. Several fields at Rothamsted have been under 

 continuous cropping for many years, and the differences in the 

 weed floras are of the most striking nature. The fields are 

 divided into plots which are manured in various ways, and the 

 differences in manuring and the time of sowing the crops have 

 a considerable influence upon the number and variety of the 

 weeds. Broadbalk field at Rothamsted has carried autumn- 

 sown wheat regularly since 1843, so that as soon as one crop 

 is harvested it is necessary to plough up the stubble in pre- 

 paration for sowing the next year's crop. Consequently, there 

 is never any opportunity of thoroughly cleaning the ground 

 by winter fallow, and as a wheat crop does not admit of much 

 cultivation in the ordinary way, a very special state of affairs 

 has risen with regard to the weeds. Black bent (Alopecurus 

 agrestis) (which occurs to some extent on many fields in the 

 district but not plentifully enough to cause any. trouble) grows 

 up with the* corn and ripens its seeds at the same time as the 

 wheat, so that harvesting operations tend to scatter the seed 

 freely on the ground. (Fig. I.) These seeds germinate at 

 once, in the autumn, and the plants develop alongside the 

 wheat, from which in their young stages they are not easily dis- 

 tinguished by casual observation. Under rotation farming the 

 weed does not get a serious hold, as it is soon cleared out by 

 the cultivation of barley and roots. On Broadbalk, however, 

 no opportunity arises of ridding the ground of the young 

 black bent plants before the wheat has to be sown, and with 

 the lapse of years the pest has spread to such a great extent 

 that it entails very great expense for hand labour to free the 

 crop in order to prevent the experiments being spoiled. It is 

 necessary to hoe out the rows in early summer, and in bad 

 seasons to hand-pull the plants in order to remove as many 

 as possible before they have a chance to seed. It is not at 

 all uncommon for heaps of black berft 3 or 4 feet high to be 

 removed from each plot of ^ acre. The strong measures 

 necessary for fighting this weed help to keep the others in 

 check, so that no other species presents such a difficult 

 problem. 



The adjoining Hoos field has carried barley every year since 

 1852, but as the barley is spring sown the weed problem is 

 far less acute. The time that elapses between harvesting in 

 August or September and sowing in the spring is sufficiently 



